Apparatus for obtaining a narrow high power laser pulse



July 21, 1970 A, DE MARlA ET AL 3,521,069

APPARATUS FOR OBTAINING A NARROW HIGH POWER LASER PULSE Filed Sept. 29.196'? 5 SheetsfSheet 1 July 21, 1970 A. J. DE MARIA ET AL 3,521,969

AflPARATUS FOR OBTAINING A NARROW HIGH POWER LASER PULSE Filed sept. 29,1967 s sheets-sheet 2 July Z1, 1970 A, DE MAR|A ET AL 3,521,069

APPARATUS FOR OBTAINING A NARROW HIGH POWER LASER PULSE Filed sept. 29,1967 5 sheets-sheet s www mm United States Patent O M 3,521,069APPARATUS FOR OBTAINING A NARROW HIGH POWER LASER PULSE Anthony J. DeMaria, West Hartford, and Albert W. Penney, Jr., Glastonbury, Conn.,assignors to United Aircraft Corporation, East Hartford, Conn., acorporation of Delaware Filed Sept. 29, 1967, Ser. No. 671,763 Int. Cl.H04b 9/00 U.S. Cl. 256-199 16 Claims ABSTRACT OF THE DISCLOSUREApparatus for generating a single, high power laser pulse having timedurations as short as "13 seconds or less in which a fast shutter suchas a Kerr cell is inserted in the path of the laser beam, either insideor outside the laser feedback cavity. The laser is simultaneouslymode-locked and Q-switched to generate a series of equally spacedpulses. One of the laser pulses is used to trigger the shutter to eitheropen or close it, depending on the configuration, for a time periodsufficient to pass only one of the laser pulses from the output of theapparatus.

A specific embodiment uses a transmission line pulse generator having aspark gap triggered by one of t-he laser pulses to trigger the shutter.

Another specific embodiment uses an optically triggered Marx-Bank pulserto trigger the optical shutter.

BACKGROUND OF THE INVENTION This invention relates to lasers, andparticularly to apparatus for obtaining a single high peak power laserpulse of time duration in the order of 10,r9 to 10'-13 seconds.

Short duration pulses up to 10'-8 seconds in width and of peak powers upto several billion watts have been generated by inserting a fasts-hutter between a laser and one of its reectors. When the shutter isclosed, excitation of the laser can be built up far beyond its normalthreshold and a high excitation is reached. When the shutter is opened,the radiation builds up rapidly and all t-he excess excitation isdischarged in an extremely short burst. Such pulses have found extremelywide usage, for example, in studying gas breakdown at opticalfrequencies, non-linear optical effects, and surface emission effects,as weil as military usage in guidance, ranging and photography.

However, the minimum pulse widths obtainable with existing techniquesare limited to approximately 10-8 seconds as a result of the requirementthat the laser pulse pass through the laser medium more than once inorder to build up the pulse.

It is apparent that there are numerous applications for single, highpeak power, Q-switched optical pulses of time durations in the range of10-9 to 10cl3 seconds in both industry and the military. For example,the fuse of an optical pulse of 101o seconds time duration would enablethe measurement of distance of a few miles to an accuracy of severalcentimeters.

SUMMARY OF THE INVENTION To accomplish the generation of such narrowpulses, it is necessary to activate an optical shutter in a time whichis short com-pared with the transit time of radiation within the opticalcavity. A series of laser pulses are generated by a simultaneouslyQ-switched and modelocked laser. The optical shutter, which ispreferably a Kerr or Pockel cell, is positioned in the path of the3,521,069 Patented July 21, 1970 ICC laser pulses. A detector is alsopositioned to respond to the laser pulses, and when the amplitude of thelaser lpulses achieves a predetermined magnitude, the Kerr cell isactuated for a time sufficient to pass one of the laser pulses.

Various configurations of the invention are described in which the Kerrcell may be positioned either inside or outside the optical feedbackcavity of the laser. Further, the Kerr cell may be opened to pass asingle laser pulse, or closed whereby a single laser .pulse is reflectedout of the laser cavity. The Kerr cell may also be opened for a timesufficient to pass two or more laser pulses if desired.

In a specific embodiment, an optically triggered transmission line pulsegenerator is used to trigger the Kerr cell.

In another embodiment, a Marx-Bank pulse generator is used to triggerthe Kerr cell.

It is therefore an object of this invention to provide a system forobtaining a single high-power laser pulse of time duration of 10-9 to10c13 seconds.

Another object of this invention is the use of an optical shutter topass a preselected number of one or more pulses from a train ofmode-locked and Q-switched laser pulses.

A further object of this invention is a fast optical shutter actuated bya train of laser pulses for producing a preselected number of one ormore laser pulses.

Another object of this invention is a transmission line pulse generatortriggered by a train of laser pulses for actuating an optical shutter.

A still further object of this invention is a Marx-Bank pulse generatortriggered by a train of laser pulses for actuating an optical shutter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of theapparatus for obtaining a single high power laser pulse in which theoptical shlutter is within the laser cavity.

FIG. 2 is a schematic diagram of a modification of FIG. 1 in which theoptical shutter is outside the cavity.

FIG. 3 shows typical mode-locked output pulses from a laser.

FIG. 4 shows schematically an optically triggered transmission linepulse generator to actuate an optical shutter outside the laser cavity.

FIG. 5 is a schematic diagram of a modification of FIG. 4 in which theoptical shutter is inside the cavity.

FIG. 6 shows schematically another embodiment of the invention utilizinga Marx-Bank pulser to trigger an optical shutter.

FIG. 7 shows the Marx-Bank pulser of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT A train of very narrow laserpulses of high peak powers may be obtained by mode-locking a laser.Modelocking may be achieved in two ways: first, by inserting a timevarying loss modulator within the lasers optical feedback cavity, themodulator having a frequency commensurate with the frequency separationof the axial modes of the laser; or, second, by the optical expandorelement behavior of saturable absorbers having a recovery time shorterthan the transit time required for an optical pulse to make one roundtrip Ibetween the two laser reflectors, the expandor element beinginserted in the optical feedback path of the laser.

The time varying loss modulator is described in U.S. Pat. 3,297,876entitled Amplitude Modulation for Lasers 'by Anthony J. De Maria, andthe saturable absorber is described and claimed in copending applicationSer. No. 536,898 entitled Self-Mode Locking of Lasers Using Dyes filedMar. 23, 1966 by Anthony J. De Maria et al., both of which are assignedto the same assignee as is this application.

The output from a mode-locked laser consists of a series of pulsesseparated in time by the round trip transit time of the optical feedbackcavity, and with pulse widths inversely proportional to the number ofaxial modes locked in synchronism. In this application the use ofsaturable absorbers for mode-locking will be employed as examples,recognizing that the use of a loss modulator is not ex cluded.

It is also known `by use of the pulse transmission mode that the peakpower output from a laser cavity may be ermanced providing the feedbackcavity is short, high reflectivity mirrors are used, and an opticalshutter or electro-optic element such as a Kerr cell in the cavity canlbe activated in a time which is short compared with the cavity transittime, See, for example, the article by A. A. Vuylsteke in the Journal ofApplied Physics, vol. 34, pp. 1'613 (1963). However, until thisinvention the problem of activating a Kerr cell in the required shorttime has not been solved. In addition, the pulse widths obtainable fromprior art pulse transmission mode systems can never be shorter than theround trip transit time of the cavity, while this invention enables oneto obtain optical pulses having widths much shorter than the transittime of the laser cavity.

Referring particularly to FIG. 1, a laser rod which for example, may beNd3+doped glass, has its ends polished at Brewsters angle, and isinserted in an optical feedback cavity consisting of mirrors M1 and M2which are dielectric coated for example, to 95% and 99% reflectivityrespectively. The mirrors of the laser cavity are spaced to produce around trip transit time of an optical pulse which is longer than thetime required to short out the M4 voltage normally maintained on theKerr cell. In our experiments, this value was typically of 16x10-9 sec.

A dye cell 12 is lled with a reversible dye solution such as Eastman9740 when a Nd3+glass is used, and is positioned in the cavity as shownto provide self-mode locking for the laser output. A polarizer 14 isalso inserted in the feedback cavity, together with a Kerr cell 18.Pumping apparatus such as a flash lamp and power supplies are requiredbut not shown. A quarter-Wave 7\/ 4) plate 16 may also be inserted inthe feedback cavity, but will not be required if a half-wave (M2)voltage is applied to the Kerr cell 18 as will be described.

Positioned adjacent output mirror M2 is a photodiode 20, a commerciallyavailable type having a fast response time, and the output of thephotodiode is fed to a thyratron 22 capable of high voltage operationand minimum anode delay time. The output of the thyratron is fed back toswitch Kerr cell 18 as Will be described.

The faces of all elements in the feedback cavity should be eitherantireection coated or oriented at Brewsters angle.

Initially a voltage is maintained on the Kerr cell 18 by the voltagesource 23 which biases the Kerr cell to its quarter-wave voltage, andthe quarter-wave plate 16 is adjusted to give maximum transmission for azero concentration of dye in cell 12. At this point the cavity is in ahigh-Q condition so long as the quarter-wave potential is across theKerr cell, and is in a low-Q condition if the Kerr cell fbias isremoved.

Dye is then added to dye cell 12 until the optical density required forself mode-locked Q-switching, with the quarter-wave potentialcontinuously applied to the Kerr cell, is obtained. The laser 10 is thenactuated by turning on the pumping apparatus, and as a result of the dyecell 12 simultaneous Q-switching and mode-locking of the laser isinitiated. A pulse of radiation bounces back and forth between mirrorsM1 and M2. This pulse increases in peak power on each pass through thelaser rod. A portion of each pulse passes through mirror M2 and theoutput resembles the pulses indicated by FIG. 3. These pulses fall onphotodiode 20. An early pulse at time T0 triggers thyratron 22. Aftersome day time A1- the thyratron shorts out the potential which has beenapplied across Keri cell 18. When this occurs, the laser pulse which ispassed to the right by polarizer 14 through quarter-wave plate 16 andKerr cell 18 will now return polarized at 90 relative to its initialpolarization rather than 1'80", and hence will be totally reected out ofthe laser cavity by the polarizer. In this manner, for a switch of thecavity Q faster than the optical round trip transit time of the cavity,almost all of the energy stored in the feedback cavity will be ejectedand no succeeding pulses will be possible because the thyratron willhold the overall Q of the feedback cavity to a low value for theremainder of the optical pumping. The delay time AT can be adjusted byadding time delay ybetween photodiode 20 and thyratron 22 so as to ejectthe pulse when it has a maximum peak power. A dye cell 25 similar to dyecell 12 may be positioned in the path of the output pulse in order topass the output pulse relatively unattenuated while absorbing thelowlevel background pulse due to the scattering of radiation in theoutput polarization by imperfections in the various optical elements inthe cavity, the nonuniform electriceld distribution across the apertureof the Kerr cell, and the switching time of the Kerr-cell circuit.

The length of the laser feedback cavity is a compromise between ease ofalignment and a high probability of selecting a single output pulse as aresult of the requirement that the switching time of the Kerr cell befaster than the optical transit time of the cavity. In the case of theapparatus described previously, the Kerr cell switching time is somewhatshorter than the round trip cavity transit time `of 16 nanoseconds. Thisswitching time produces a single output pulse approximately 75% of thetime. Using the above described configuration, single pulses of 0.2joule and 0.8 nanosecond half-amplitude duration have been obtained.

FIG. 2 shows a modification of the system `of FIG. l in which theoptical shutter, i.e. the Kerr cell, is positioned outside the laserfeedback cavity. The laser 30 is inserted in a feedback cavitycomprising mirrors M3 and M4. A dye cell 32 is positioned in the cavityas previously. Outside the cavity but in the path of the laser beam is apolarizer 34 and a Kerr cell 38. Adjacent mirror M3 and also in linewith the laser beam at the other side of the cavity is an outputdetector such as photodiode 40. The output of the photodiode is fed to athryratron 42 as described previously. A voltage source 44 maintains theKerr cell 38 biased to its full-wave voltage. By making the reflectivityof M4 less than M3, the major output of the laser is to the right of M4.The output from the right end of the laser through the Kerr cell 38 andpolarizer 34 constitutes the lasers simultaneously mode-locked andQ-switched output. The small output from the left end of the laser isallowed to proceed through mirror M3 and impinge on photodiode 40. Anearly pulse occurring at time To (see FIG. 3) triggers thyratron 42.After some time delay Ar, the thyratron shorts out the .potentialapplied across Kerr cell 38. The polarization of the pulse occurring attime fr, is now rotated with respect to the previous pulses. Thepolarizer 34 ejects the pulse occurring at time 1-1. As a consequencethis pulse does not appear in the series pulses to the right ofpolarizer 34. If the voltage is reapplied to the Kerr cell after timef1, the subsequent pulses pass through polarizer 34. The time duringrwhich the potential across Kerr cell 38 is shorted out by the thyratron42 can be made as large as one chooses, and one, two or more pulses canbe ejected from the pulse train by this arrangement.

The distance between mirrors M3 and M4 is chosen so as to have themode-locked pulse repetition rate longer than the opening and closingtime of Kerr cell 38. Before the occurrence of the next lasermode-locked pulse, the Kerr cell is fully opened and the next laserpulse proceeds through the shutter of the Kerr cell. If only one outputpulse is desired, the shutter of the Kerr cell can be allowed to closeby the time of the occurrence of the next laser pulse, therebypreventing the passage of additional pulses. The time interval cantypically be anywhere from 2 to 30x10*9 seconds.

It is obvious to those skilled in the art that the Kerr cells of FIGS. land 2 may be replaced by Pockel cells with a corresponding adjustment inthe amplitude and polarity of the cell opening and closing signals.

FIG. 3 shows a series of normal mode-locked output pulses from a lasersuch as occur in FIGS. 1 and 2. The delay time A1- may be chosen so thatthe Kerr cell opens after the pulse occurring at time T1, therebyejecting the pulse having the largest amplitude, i.e. the pulseoccurring at 1-2. The pulse at time T2 thereupon passes through the Kerrcell. By time T3, the Kerr cell is closed preventing the passage ofadditional pulses. As a result only the pulse occurring at time 1-2passes through to the output.

FIGS. 4 and 5 show how a single narrow laser pulse may be produced bythe use of an optically triggered transmission line pulse generator usedto trigger a Kerr cell in the optically biased pulse transmission mode.The transmission line pulser can generate up to 2O to 40 kilovolt pulsesof 20 nanosecond time duration and 1 to 2 nanosecond rise and falltimes.

Recent work has proven that it is possible to trigger a spark gap with amode-locked laser pulse and have a delay time of considerably less thannanoseconds since the rise time of a mode-locked laser is in thesub-nanosecond range. It is not necessary to produce visible opticalbreakdown of a gas such as SFS or argon in the interelectrode gap toeffectively trigger the spark gap.

Referring to FIG. 4, the transmission line pulse generator consists oftwo sections of coaxial transmission line 50 and 52 of the samecharacteristic impedance. A high voltage input is generated from asource, not shown, and connected through low capacitance, high voltageresistor R1 to the pulse forming section 50 of the pulse generator. Thefirst section of the line 50 is charged to a voltage V1 with a timeconstant R1C1, where C1 is the total capacity of the first section ofthe transmission line 50. Once charged, no further current flows inresistor R1 and the pulse forming section 50 is maintained at a constantpotential V1.

Between the pulse forming section 50 and the delay line section 52 ofthe transmission line pulse generator, a gap exists. The entire pulsegenerator is packaged within a pressurized container 54 and filled witha gas such as SFS or argon. In the gap between the two sections 50 and52, lenses L1 and L2 are inserted in the container 54. The gap betweenthe two transmission line sections is also pressurized.

If a small value of electric eld strength to pressure ratio is chosenfor the gap between sections 50 and 52, the gap will hold off thepotential V1 indefinitely. However, for very large values of electricfield strength to pressure ratio, the gap will break down after knowndelay time. Thus if potential V1 is applied through resistor R1 rapidlyenough, and an optical trigger pulse applied in the gap in a time shortcompared to the delay of the gap for a given electric eld to pressureratio, the gap breakdown can be controlled with the optical pulse.

Once the gap is triggered, the pulse :forming section 50 is electricallyconnected to the delay section 512. Since the two sections of the linehave the same characteristic irnpedance, the voltage will divide equallybetween the two lines and two traveling waves will be produced. Onhall:` of the voltage V1 will travel to the left and the other half willtravel to the right. The 'wave traveling to the left in the pulseforming section 50 reects from the end of the line as though it were anopen circuit because resistor R1 is very large relative to the impedanceof the transmission line, and its capacitance is very low.

The open circuit reflection produces another wave having a voltage V1/2traveling to the right thus leaving the line to the left of ituncharged. The pulse forming section thus supplies a voltage pulse oftime duration tp 6 equal to twice the length of the line divided by thevelocity of light.

At the right hand side of the delay line 52, a resistor R2, Kerr cell 56and Resistors R3 and R1 are positioned as shown. Windows W1 and W2 areplaced in the walls of container S4 in alignment with Kerr cell 56.

The pulse originally started to the right in the delay section 52continues to travel to the right until it arrives at the junction of thetransmission line 52 and resistors R2 and R4. The values of R2, R3 andR1 are chosen to match the Kerr cell 56 to the characteristic impedanceof the transmission line. If properly matched, the transmission linewill produce no reflection and a potential of V1/ 2 will be producedacross the Kerr cell 56 for the duration tp of the traveling wave. Ifthe Kerr cell voltage for quarter wave retardation of a laser pulse isV1/ 2, the optical path through the Kerr cell 56 will be open for a timeequal to t1, which is chosen to be approximately equal to themode-locked pulse repetition period of the laser.

Output pulses from a mode-locked laser `58 are directed through a halftransmitting-half reflecting mirror M1 where the transmitted portion ofthe pulses pass through lens L1 and through the gap between the twosections 50 and 52 of the transmission line. At time t0, ionization isproduced at the focal point of lens L1 at the center of the spark gap.The laser radiation in the gap initiates a set of traveling waves attime t1=t0lAt, where At is the gaps trigger initiation delay time.

A portion of the mode-locked pulses are reflected from mirror M1, andreflected from mirrors, M2, M3, M4 through polarizer P and window W1.The pulses then travel through the Kerr cell 56, the window W2, thequarter wave plate WP, and are reflected by mirror M5 back to thepolarizer P where the pulses are then reflected out of the system whenthe Kerr cell optical switch is closed.

Referring to FIG. 3, if the gap between sections 50 and 52 of thetransmission line is triggered by pulse f2, and if the optical delay viamirrors M1, M2, M3 and M4 is slightly greater than the electrical delayin the transmission line pulser, each pulse in FIG. 3 except for pulset2 will be reected outside the system. The mirror separation dimension lis made adjustable so that the optical delay can be varied toaccommodate any value of trigger delay time At.

When the Kerr cell 56 optical switch is opened by the transmission linepulse, the radiation previously reected outside the system by polarizerP now passes directly through the polarizer and along the path M4, M3,M2 and M1 to produce an output pulse. In the case illustrated, the pulsewhich occurs, at time t2 will represent the output of the system.

FIG. 5 is basically similar to FIG. 4 and illustrates the transmissionline Kerr cell pulser for selecting a high amplitude pulse from asimultaneously Q-switched and mode-locked laser. However, in this casethe Kerr cell KC, Glenn-Thomson prism polarizer P and the quarter waveplate WP are positioned in the optical feedback path of the laser.Mirrors M6 and M7 form the laser feedback cavity, whereas mirrors M8 andM9 reect the laser pulse through lens L2 to the gap between the sectionsof the transmission line pulse generator.

The two end capacitors of the transmission line pulser C1 and C2 arecharged to a voltage V1. The capacitor C1 maintains a voltage V1 acrossthe Kerr cell KC equal to the quarter wave retardation voltage. Thelaser is then excited and simultaneously Q-switched and mode-locked bythe dye cell 60` as previously explained.

As the laser radiation proceeds to bounce back and forth betwen thereflectors, it is polarized by the prism P', converted into circularpolarization by the quarter wave plate WP', polarized by an additionalquarter wave retardation by the Kerr cell KC, reflected from partialmirror M7, reconverted to circular polarization by the Kerr cell, andreconverted to the polarization required for transmission through theprism lby the quarter Wave plate WP.

The output of the laser from mirror M7 is directed to the lens L1 bymirrors M8 and M9. Lens L1 focuses the laser radiation in the center ofthe pressurized spark gap. When the radiation is suiiiciently intense,the gap breaks down and after a delay of t seconds determined by thelength of the section of transmission line, both sides of the Kerr cellhave the same potential and thus the Kerr cell optical switch is turnedoff.

Radiation directed toward the prism P from mirror M7 is now rotated 90with respect to the incoming radiation, and is thus reiiected out of theoptical feedback cavity of the laser by the prism P. This coupled outradiation produces the output of the laser. The laser is then turned offand ceases to oscillate.

Another technique for energizing Kerr cell or Pockel cell for selectingone of the higher peak power subnanosecond pulses occurring in theoutput of a simultaneously Q-switched and mode-locked laser is shown inFIG. 6. A laser 10 such as a Brewster ended Nd3+ doped glass rod ispositioned between two 99%+ reliectivity mirrors M1 and M2. Typicallythe laser is 53 cm. long and 1.3 cm. in diameter, and mirrors M1 and M2are separated by l meter.

A dye cell 72 is positioned inside the laser cavity to providemode-locking and Q-switching as described previously. A polarizer 74 anda Kerr cell 76 are also placed within the optical cavity.

A Marx-Bank pulser 78 is energized by a power supply 80 and the outputfrom the puiser energizes Kerr cell 76.

Initially the Kerr cell is unenergized, and the polarizer 74 is adjustedfor maximum transmission. Zero initial voltage condition is optimum forobtaining a uniform rejection ratio across a Kerr cell aperture.

Pumping is applied to the laser to produce lasing action, and dye cell72 acts as described as a passive modulator having a frequency equal tothe reciprocal of the round trip time required for an optical pulse totraverse the distance between the two reectors M1 and M2, typically 7.5nanoseconds. This modulating action produces ultrashort pulses of lighthaving a pulse width equal to the reciprocal of the gain line width ofthe laser medium, and an amplitude resembling a typical Q switch typeenvelope.

The leakage laser radiation from polarizer 74 is focused onto the firstgap 84 of the Marx-Bank pulse generator 78 by means of lens 82. TheMarx-Bank generator is shown in detail in FIG. 7. At a predeterminedpulse amplitude, the laser energy focused through lens 82 to the sparkgap breaks down the spark gap, thereby causing rapid succesive breakdownof the remaining overenergized spark gaps 84. Each gap breakdowneffectively places one of a series of 6 parallel charged capacitors inseries with other capacitors. Pulse generators of this type have onlyapproximately a nanosecond delay from the initial gap breakdown to theappearance of a high voltage across the load, in this case, Kerr cell76.

When all the gaps 84 are broken down a fast rise time,

volt, quarter wave voltage pulse is applied across the.

Kerr cell 76. With the Kerr cell energized, the polarization of thepulse is rotated for each pass through the Kerr cell, and subsequentlyejected out of the laser cavity by polarizer 74. The radiation ejectedby polarizer 74 constitutes the laser output. The long RC time constantof the Kerr cell, Marx-Bank circuit maintains the Q of the laser cavityat a low value for suflicient time to prevent succeeding pulses frombeing initiated. It is also understood that avalance transistors may beutilized in place of spark gaps in the Marx-Bank. A photodetector isthen utilized to detect the leakage radiation from lens 82. The signalfrom the photodetector is then utilized as a signal input to theMarx-Bank.

Although this invention has been shown and described with respect to thelpreferred embodiments thereof, it is understood that numerous changesmay be made without departing from the scope of this invention, which isto be limited and defined only by the following claims.

Having thus described the preferred embodiment of our invention, what weclaim as new and desire to secure by Letters Patent of the United Statesis:

1. Apparatus for producing a single narrow high-power laser pulsecomprising:

a laser, means for producing from said laser a train of simultaneouslymode-locked and Q-switched laser pulses,

optical shutter means and polarizer means positioned in the path of saidpulse train for passing said laser pulse train therethrough,

a transmission line,

means for electrically connecting said transmission line to said opticalshutter means,

means for generating a voltage in said transmission line, meansresponsive to said laser pulse train for causing said voltage to be fedto said optical shutter means to vary the optical transmissionproperties thereof,

and means responsive to a variation in said optical shutter transmissionproperties for varying the polarization of at least one of the laserpulses passing therethrough whereby said pulses are isolated from saidpulse train upon passing through said polarizer means.

2. Apparatus as in claim 1 in which said optical shutter means is a Kerrcell which is positioned outside the optical feedback cavity of saidlaser.

3. Apparatus as in claim 2 in which said transmission line is sealed ina pressurized container and includes a pulse forming section and a delayline section,

-a gap between said transmission line sections,

means for generating a voltage in said pulse forming transmission linesection,

leans means in said container for focusing said laser pulse train onsaid gap, said gap breaking down when the amplitude of said laser pulsesreaches a predetermined magnitude whereby a voltage pulse is produced insaid delay line section,

and means for transmitting said voltage pulse to said Kerr cell to varythe optical transmission properties thereof.

4. Apparatus as in claim 2 and including,

mirror means for directing said laser pulse train through said polarizerand through said Kerr cell in a first direction and for reiiecting saidlaser pulse train back through said Kerr cell and said polarizer in asecond direction opposite said first direction, means for initiallybiasing said Kerr cell to a condition of operation in which said laserpulses are polarized in a manner that said laser pulses will not passthrough said polarizer in said second direction, and means includingsaid voltage pulse produced in the said transmission line for varyingthe optical transmission properties of said Kerr cell to polarize atleast one laser pulse in a manner which will allow said pulse to passthrough said polarizer in said second direction. l

5. Apparatus as in claim 1 in which said optical shutter means is a Kerrcell which is positioned withln the optical feedback cavity of saidlaser.

6. Apparatus as in claim 5 and including first and second capacitors,said Kerr cell being positioned between said first capacitor and one endof said transmission line,

said second capacitor being positioned at the other end of saidtransmission line and separated from said transmission line by a gap,

means for charging said capacitors to a predetermined voltage,

means connecting said first capacitor to said Kerr cell to bias saidKerr cell to a condition of operation wherein said laser pulses passthrough said Kerr cell and said polarizer,

means including a lens in said container for focusing said laser pulseson said gap, said gap breaking down when said laser pulse amplitudereaches a prede termined magnitude whereby a voltage pulse is generatedin said transmission line,

and means connecting said voltage pulse to said Kerr cell to bias saidKerr cell to a condition of operation whereby at least one of said laserpulse is rellected from said polarizer.

7. Apparatus for isolating one or more laser pulses from a train ofpulses comprising a laser medium rst and second mirrors positioned atopposite ends of said laser medium forming an optical feedback cavityfor said laser,

means for energizing said laser to produce a train of laser pulses,

optical shutter means positioned in the path of said laser pulse trainfor passing said lasser pulses therethrough, means for controlling theoptical transmission properties of said optical shutter means to causethe laser pulses passing therethrough to be in either a rst or a secondpolarized state,

polarizing means positioned in the path of said laser pulses, saidpolarizing means passing said laser pulses therethrough when said pulsesare in said iirst polarized state, and rellecting said pulses out ofsaid polarizing means when said pulses are in said second polarizedstate,

means positioned to receive a selected portion of said laser pulses andresponsive to the occurrence of said laser pulses for producing asignal,

and means feeding said signal to said optical shutter control means tovary said control means and change the optical transmission propertiesof said optical shutter means for a time suicient to pass at least oneof said pulses therethrough and cause the laser pulses passingtherethrough to be changed from one of said rst or second polarizedstates to the other state whereby said pulses in said changed polarizedstate are isolated from said pulse train.

8. Apparatus as in claim 7 in which said means for controlling saidoptical shutter means includes a voltage supply for applying anelectrical voltage thereto.

9. Apparatus as in claim 8 in which said means for producing a signalincludes a detector positioned to receive a selected portion of thepulses generated by said laser,

and thyratron means responsive to the output from said detector forgenerating said signal, said signal being fed to said voltage supply tovary the voltage applied to said optical shutter control means.

10. Apparatus as in claim 7 and including a transmission line having oneend connected to said optical shutter means,

a source of voltage,

and means responsive to said laser pulses for selectively connectingsaid voltage source with said transmission line when said laser pulsesreach a predetermined amplitude whereby a voltage is applied to saidoptical shutter means.

11. Apparatus as in claim 7 in which said laser pulse train consists ofa train of simultaneously mode-locked and Q-switched laser pulses.

12. Apparatus as in claim 7 and including a qaurter wave platepositioned in the path of said laser pulse train.

13. Apparatus as in claim 7 in which said optical shutter means and saidpolarizing means are positioned within the optical feedback cavity ofsaid laser.

14. Apparatus as in claim 7 in which said optical shutter means and saidpolarizing means are positioned outside the optical feedback cavity ofsaid laser.

1S. Apparatus as in claim 7 in which said optical shutter means is aKerr cell.

16. Apparatus as in claim 7 in which said optical shutter means is aPockel cell.

References Cited UNITED STATES PATENTS 3,277,393 10/ 1966 Nicolai.3,395,367 7/1968 Bell et al. 331-945 3,423,695 1/1969 Boyden 350-160OTHER REFERENCES H. A. Heymau: Proceedings of IEEE, An Improved Mode ofKerr 'Cell Operation, v. 53' No. 12, December 1965.

N. George: Applied Optics, Farraday Rotators for High Power LaserCavities, v. 5, No. 7, July 1966, pp. 1183- 1185.

I. I. Masters: Electronics, Q-switching, Oc-tober 1965, pp. 91-95.

ROBERT L. GRIFFIN, Primary Examiner A. J. MAYER, Assistant Examiner U.S.Cl. X.R.

